1. Field of the Invention
The invention relates to an apparatus used to secure an offshore buoyant tower structure to the ocean floor.
2. Description of the Prior Art
In the positioning of a floatable or semi-submersible marine structure in an offshore body of water, the method normally followed is to anchor the structure by use of anchors and lines which radiate downwardly from the structure. In the instance of a self-supporting or buoyant tower structure for use in deeper water, however, with a single upstanding column-like member that is controllably buoyant in the water, the lower portion of such a buoyant tower member must be firmly anchored to the ocean floor such that the column will, under the influence of wind, waves and other elements, be permitted only a limited degree of oscillatory movement about a fixed lower end.
To achieve such a purpose, the lower or base end must be so firmly embedded or weighted to the ocean floor that, in effect, it will tether the buoyant tower structure to the ocean floor. Anchoring can assume a number of forms presently known and including primarily the use of vertical piling which is normally embedded downward into the substratum a sufficient distance to be held in position by the subsoil. To offset the tower's buoyancy ballast may also be used near the base of the tower. In any event, the piles must also resist the lateral movement of the buoyant column and supplementary "shear piles" may be added for this purpose.
As described in U.S. Pat. No. 3,488,967, entitled "Combination Deep Water Storage Tank and Drilling and Production Platform", filed Mar. 23, 1967, and subsequently issued to Mr. M. Toossi, Jan. 13, 1970, a circular arrangement of vertical piles can also be used to form the column structure of the buoyant tower. Wells may also be drilled vertically downward through these piles, though it is well recognized that use of a curved or inclined pile to direct the well into the substratum would allow the well to be drilled into more distant hydrocarbon bearing formations.
Due to lateral and rotational movement of the buoyant structure means must also be used at the ocean floor to reduce the resultant shear and bending stresses encountered in the piles. As described in U.S. Pat. No. 3,648,638, entitled "Vertically Moored Platforms", filed Mar. 9, 1970, and subsequently issued to Mr. K. Blenkarn, Mar. 4, 1972, ball joints may be incorporated into the piles adjacent the ocean floor, or a ball joint may be used in the column, above the piles.
But use of these ball joints necessarily increases the cost and complexity of assembly of the underwater structure and leaves the integrity of the structure dependent upon the ball joints' continued operation.
An apparatus needs to be developed therefore that is not dependent upon the proper operation of mechanical stress compensation devices. The same apparatus should also not limit the drilling of wells to formations disposed vertically beneath the structure.